1. Field of the Invention
This invention relates to gas turbine engines. More particularly but not exclusively this invention relates to a gas turbine engine afterburner.
2. Description of Related Art
Afterburning is a method of augmenting the basic thrust of a gas turbine engine to improve aircraft take-off, climb and in the specific case of military aircraft, combat performance. Additional fuel is introduced and burned between the turbine section of the engine and the jet pipe propelling nozzle utilising unburned oxygen in the exhaust gas to support combustion. The resulting increase in the temperature of the exhaust gas gives an increased velocity to the jet leaving the jet pipe nozzle and therefore increases the engine thrust.
Although the gas temperature in the jet pipe is extremely hot the afterburner cannot be relied upon to ignite spontaneously. Some form of ignition has to be provided, therefore, for reliable operation.
In hot-shot ignition a jet of fuel is injected into the combustion chamber outlet. The resulting hot streak of flame extends through the turbine into the jet pipe where it ignites the afterburner fuel fed into the jet pipe. This form of afterburner ignition necessitates the use of at least one fuel injector at least the tip of which extends into the combustion chamber.
A problem arising with the use of hot shot ignition is that carbon debris left by burnt or boiling fuel in the injector rapidly builds up. This debris has to be removed otherwise the injector becomes blocked and ceases to function. Regular examination and frequent preventative maintenance of the injectors is thus required. Unless the injectors are easily accessed and removed this could mean the aircraft has to be taken out of service.
It is believed that the generation of carbon debris is temperature related and its build-up occurs in the injectors delivery passage lying in the space between the combustion chamber outer casing and the wall of the combustion chamber where the temperature may be of the order of 130° C. which is considerably less than the temperature of for example the area in which the combustion chamber where the nozzle is positioned and which temperature may be of the order of 1,300° C. The temperature gradient along the injector delivery passage has been found to be very steep and it is believed that this may be a primary cause of carbon debris build up. It is also believed that control of the thermal gradient as by ducting air over the injector or even by applying some form of thermal lagging to the body of the injector is likely to greatly retard the build up of such debris.